Securely store hashed, salted passwords. If you need to store and verify
passwords, there are many wrong ways to do it, most of them all too
common. Some people store users' passwords in plain text. Then, when an
attacker manages to get their hands on this file, they have the passwords for
every user's account. One step up, but still wrong, is to simply hash all
passwords with SHA1 or something. This is vulnerable to rainbow table and
dictionary attacks. One step up from that is to hash the password along with
a unique salt value. This is vulnerable to dictionary attacks, since guessing
a password is very fast. The right thing to do is to use a slow hash
function, to add some small but significant delay, that will be negligible
for legitimate users but prohibitively expensive for someone trying to guess
passwords by brute force. That is what this library does. It iterates a
SHA256 hash, with a random salt, a few thousand times. This scheme is known
as PBKDF1, and is generally considered secure; there is nothing innovative
happening here.

The API here is very simple. What you store are called password hashes.
They are strings (technically, ByteStrings) that look like this:

Each password hash shows the algorithm, the strength (more on that later),
the salt, and the hashed-and-salted password. You store these on your server,
in a database, for when you need to verify a password. You make a password
hash with the makePassword function. Here's an example:

This will hash the password "hunter2", with strength 12, which is a good
default value. The strength here determines how long the hashing will
take. When doing the hashing, we iterate the SHA256 hash function
2^strength times, so increasing the strength by 1 makes the hashing take
twice as long. When computers get faster, you can bump up the strength a
little bit to compensate. You can strengthen existing password hashes with
the strengthenPassword function. Note that makePassword needs to generate
random numbers, so its return type is IOByteString. If you want to avoid
the IO monad, you can generate your own salt and pass it to
makePasswordSalt.

Your strength value should not be less than 10, and 12 is a good default
value at the time of this writing, in 2011.

Once you've got your password hashes, the second big thing you need to do
with them is verify passwords against them. When a user gives you a password,
you compare it with a password hash using the verifyPassword function:

These two functions are really all you need. If you want to make existing
password hashes stronger, you can use strengthenPassword. Just pass it an
existing password hash and a new strength value, and it will return a new
password hash with that strength value, which will match the same password as
the old password hash.

Registering and verifying passwords

Hash a password with a given strength (12 is a good default). The output of
this function can be written directly to a password file or
database. Generates a salt using high-quality randomness from
/dev/urandom or (if that is not available, for example on Windows)
Random, which is included in the hashed output.

Updating password hash strength

Try to strengthen a password hash, by hashing it some more
times. strengthenPassword pwHash new_strength will return a new password
hash with strength at least new_strength. If the password hash already has
strength greater than or equal to new_strength, then it is returned
unmodified. If the password hash is invalid and does not parse, it will be
returned without comment.

This function can be used to periodically update your password database when
computers get faster, in order to keep up with Moore's law. This isn't hugely
important, but it's a good idea.

Utilities

A salt is a unique random value which is stored as part of the password
hash. You can generate a salt with genSaltIO or genSaltRandom, or if you
really know what you're doing, you can create them from your own ByteString
values with makeSalt.

Generate a Salt with 128 bits of data taken from a given random number
generator. Returns the salt and the updated random number generator. This is
meant to be used with makePasswordSalt by people who would prefer to either
use their own random number generator or avoid the IO monad.